Intensities of Linear-Molecule Vibration-Rotation Transitions with |Deltak| = 2, with Applications to HCN, DCN, and HCCH
The Q branch of the 2nu22-0(0) band of the HCN molecule and an intensity asymmetry between the P and R branches have recently been observed by Maki, Quapp, Klee, Mellau, and Albert (J. Mol. Spectrosc. 185, 356-369 (1997)). Such intensity effects are associated with a contribution from the &Mtilde;22 terms of the effective dipole moment operator of Aliev and Watson. The quantum number dependence observed by Maki et al. (cited above) is in agreement with this contribution. A formula is derived for the coefficient of this term, and the value calculated from the anharmonic potential and dipole derivatives is in good agreement with the observed value for a particular choice of signs of the combination-band transition moments R12 and R32. For DCN the second derivatives of the dipole moment are not known, but a comparison with the HCN calculation suggests that the agreement is adequate in view of the relatively large uncertainty of the observed values. Similar intensity effects should occur in the (nu4 + nu5)2-0(0) band of HCCH, for which the coefficient is calculated to be similar in magnitude to that of the 2nu22-0(0) band of HCN. The intensity of the Sigma--Sigma+ component of (nu4 + nu5)-0 of HCCH, which should consist of only a Q branch, is also discussed; from present knowledge of the molecular parameters, it is expected to be much weaker than the Q branch of the Delta-Sigma+ component. Copyright 1998 Academic Press.
Watson
JKG
Steacie Institute for Molecular Sciences, National Research Council of Canada, Ottawa, Ontario, K1A 0R6, Canada